• If it ends in a vowel, add -s. Examples: verde ("green," singular), verdes ("green," plural). El árbol es verde, the tree is green. Los árboles son verdes, the trees are green.
• If it ends in a z, change the z to a c and add -es. Example: feliz ("happy," singular), felices ("happy," plural). Soy feliz, I'm a happy person; somos felices, we are happy people.
• If it ends in another consonant or a stressed vowel, add -es. Example: difícil ("difficult," singular), difíciles ("difficult," plural). La tarea es difícil, the task is difficult; las tareas son difíciles, the tasks are difficult.

Making a masculine adjective feminine is even easier. Just follow these steps:

• If the singular masculine adjective ends in an -o, change it to an -a. Example: pequeño ("small," masculine singular), pequeña ("small," feminine singular). El gato es pequeño, the cat is small; los gatos son pequeños, the cats are small; la chica es pequeña, the girl is small; las chicas son pequeñas, the girls are small.
• If the singular masculine adjective ends in any other letter, the feminine form is the same. El autobús es grande, the bus is big; la casa es grande, the house is big.

Important note: Invariable adjectives don't change in form and are very rare. Most of them are either uncommon colors (AZUL, VERDE) or words of foreign origin. An example is web as in la página web (the web page) and las páginas web (the web pages).